1. Field of the Invention
The present invention relates to a radio IC card system using a digital data communication system and a radio IC card, and more particularly relates to a radio IC card system and radio IC card arrangements not requiring the use of a delay line or a phase-locked loop (PLL).
2. Description of Related Art
In a card system such as a commuter ticket, in addition to a contact and intimate contact-type system such as a magnetic card system, a non-contact-type system (e.g., activated by radio) has become of interest. That is to say, electric power required for operation is supplied to a card by radio transmission (thus advantageously not requiring a battery), and an electronic circuit on the card is supplied with electric power from such transmission, and as a result thereof, performs communicating with a main controller.
In the above described system, the controller modulates a high frequency signal by information to be transmitted, and sends it from a fixed (e.g., wall-mounted) base unit via air transmission using an antenna. Energy sent is received by an antenna of a mobile (i.e., card) unit, and information is reproduced by a demodulating circuit on the card. The high-frequency signal is called a carrier wave. Several types of modulation/demodulation systems are known for conveying information, i.e., known are amplitude shift keying (ASK) in which an amplitude of the carrier wave is shifted according to digital information, frequency shift keying in which a frequency of the carrier wave is shifted (FSK), and phase shift keying (PSK) in which a phase of the carrier wave is shifted. PSK is best because it has a low signal error rate and a narrow occupation frequency bandwidth, and therefore it has been used in many modulation/demodulation systems for digital data transmission. A delay detection system using a delay line and a detection system in which a synchronous clock is generated by a phase-locked loop has been known as when the information is demodulated by PSK. However, use of a delay line and phase-locked loop is disadvantageous in that such adds to a complexity, size, cost and power consumption of the system.